This invention is directed to a multilevel pulse width modulated inverter and, in particular, to a thyristor-inverter having a wide range of voltage control with harmonic elimination.
The ever increasing use of static power converters in variable speed motor devices is highlighting the problem of harmonic distortion in the voltage and current waveforms of present adjustable voltage, adjustable frequency square wave inverters.
Presently, harmonic reduction is achieved by three basic methods: (a) multilevel waveforms, (b) sinusoidal pulse-width modulation, and (c) pulse width control.
Multilevel operation can be achieved by summing the outputs of several inverters operating in parallel through phase shift transformers as described in U.S. Pat. No. 3,324,374, which issued on June 6, 1967, to P. D. Corey. This technique requires large reactive elements and is no longer attractive. Recently, as described in U.S. Pat. No. 4,135,235, which issued to R. H. Baker on Jan. 16, 1979, transistor inverters with multilevel voltage waveforms have been proposed. While providing an efficient reduction in harmonic distortion, the inverter has limited power levels due to the ratings of available power transistors.
By chopping a voltage under the control of a sinusoidal signal, as described in U.S. Pat. No. 4,047,083, which issued on Sept. 6, 1977, to A. Plunkett, all dominant harmonics can be eliminated, however, only partial power can be obtained. By producing several pulses during each half-period of the output waveform and by controlling their width, selected harmonics can be eliminated from the inverter output. However, such an inverter is still derated and voltage control is limited.